We are using lanthanide ions as probes of calcium binding sites in proteins and enzymes. By substituting lanthanide ions for calcium, it is possible to use nmr and fluorescence techniques to measure distances between two sites in the proteins. We are measuring distances from the active center to the metal binding site for trypsins of various origin. We also plan to map the active site of thermolysin and measure distances from various protons on a competitive inhibitor to the two different type of metal binding sites in the protein. BIBLIOGRAPHIC REFERENCES: D.W. Darnall and E.R. Birnbaum, "The Metal Ion Acceleration of the Activation of Trypsinogen to Trypsin," a review in Metal Ions in Biological Systems, Vol. 6, Edited by Helmut Siegel, Marcel Dekker, 1976. D.W. Darnall, E.R. Birnbaum, F. Abbott, J. Gomez, F. Eidson and B. Garcia, "An Estimation of the Distance from the Active Site to the Calcium Binding Site in Trypsin," in Proteolysis and Physiological Regulation, D.S. Ribbons and K. Brew, Eds., Miami Winter Symposium, Academic Press, 11, 394, (1976).